Substitution of seleno-methionine for methionine is bacterially expressed proteins has proved to be a highly successful method for solving the "phase problem" in determining the x-ray structures of proteins. Unfortunately, little is known at present about the chemistry of seleno-methionine in proteins. For example, the propensity for oxidation of seleno-methionine is unknown and so therefore is the stability of crystals of seleno-methionine-containing proteins. We are thus undertaking a major effort to elucidate the chemistry of seleno-methionine in peptide model systems as well as in the protein human chorionic gonadotrophin. Thus far, mass spectrometric analysis have revealed a number of interesting and previously undescribed findings that promise to be important for understanding the chemistry of seleno-methionine containing systems. A paper describing this work is in preparation.